Messenger-RNA-binding proteins and the messages they carry

Eukaryotic messenger RNAs require information that specifies their nuclear export, subcellular localization, translation and stability, in order to function properly. It is becoming apparent that this information is provided not only by the mRNA sequence, but also by specific RNA-binding proteins - collectively referred to as heterogeneous nuclear ribonucleoproteins (hnRNP proteins) or mRNA-protein complex proteins (mRNP proteins). Most of the hnRNP proteins associate with the pre-mRNAs co-transcriptionally, whereas others associate later as a consequence of the processing reactions that form the mRNAs. Many hnRNP proteins are removed with the excision of introns during splicing, and the spliceosome clears away proteins from the vicinity of exon-exon junctions, leaving the exon-exon junction complex (EJC) in its wake. However, many of the hnRNP proteins remain on the mRNAs after splicing and, together with the EJC proteins, accompany the mRNAs to the cytoplasm. The specific constellation of proteins on a given mRNA carries an impression of its childhood experiences and profoundly influences its fate. What results is an mRNP that is much richer in information than the sequence of the mRNA itself. Because several proteins are bound at exon-exon junctions after export to the cytoplasm, they provide a molecular memory that documents the overall structure of the pre-mRNA, and communicates crucial information to the translational machinery for the surveillance of nonsense mutations and for mRNA localization and translation.